As a result of the Montreal protocol phasing out ozone depleting chlorofluorocarbons (FCs) and hydrochlorofluorocarbons (HCFCs), industry has been working for the past few decades to find replacement refrigerants. The use of new hydrofluorocarbons in various products, such as solvents, blowing agents, cleaning agents, aerosol propellants, heat transfer media, dielectrics, fire extinguishants and power cycle working fluids has made hydrofluorocarbons the subject of considerable interest, especially since they have reduced global warming potential (GWP) and reduced ozone depletion potential (ODP). Promising hydrocarbons include hydrofluoroolefins.
Hydrofluoroolefins (HFOs), such as tetrafluoropropenes (including 2,3,3,3-tetrafluoropropene (HFO-1234yf)), are now known to be effective refrigerants, fire extinguishants, heat transfer media, propellants, foaming agents, blowing agents, gaseous dielectrics, sterilant carriers, polymerization media, particulate removal fluids, carrier fluids, buffing abrasive agents, displacement drying agents and power cycle working fluids. Unlike chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), both of which potentially damage the Earth's ozone layer, HFOs do not contain chlorine and, thus, pose no threat to the ozone layer. In particular, 2,3,3,3-tetrafluoropropene (HFO-1234yf or 1234yf) has been identified as a suitable compound having zero ODP and low GWP. HFO-1234yf has also been shown to be a low global warming compound with low toxicity and, hence, can meet increasingly stringent requirements for refrigerants in mobile air conditioning. Accordingly, compositions containing HFO-1234yf are among the materials being developed for use in many of the aforementioned applications.
Several methods of preparing HFOs, including HFO-1234yf, are known. For example, U.S. Pat. No. 4,900,874 (Ihara et al) describes a method of making fluorine containing olefins by contacting hydrogen gas with fluorinated alcohols. Although this appears to be a relatively high-yield process, commercial scale handling of hydrogen gas at high temperature is hazardous. Also, the cost of commercially producing hydrogen gas, such as building an on-site hydrogen plant, is economically costly.
U.S. Pat. No. 2,931,840 (Marquis) describes a method of making fluorine containing olefins by pyrolysis of methyl chloride and tetrafluoroethylene or chlorodifluoromethane. This process is a relatively low yield process and a very large percentage of the organic starting material is converted to unwanted and/or unimportant byproducts, including a sizeable amount of carbon black which tends to deactivate the catalyst used in the process.
The preparation of HFO-1234yf from trifluoroacetylacetone and sulfur tetrafluoride has been described (See Banks, et al., Journal of Fluorine Chemistry, Vol. 82, Iss. 2, p. 171-174 (1997)). Also, U.S. Pat. No. 5,162,594 (Krespan) discloses a process wherein tetrafluoroethylene is reacted with another fluorinated ethylene in the liquid phase to produce a polyfluoroolefin product.
One method for producing HFO-1234yf uses 1,1,2,3,-tetachloropropene (HFO-1230xa or 1230xa) as a starting material. In a method, HCO-1230xa can be catalytically converted to 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf) in the presence of hydrogen fluoride (HF). HCFO-1233xf can then be converted to HFO-1234yf. One such process for preparing HFO-1234yf is described in U.S. Patent Application No. 2007/097842, the contents of which are incorporated by reference. A process for producing HFO-1234yf is also described in U.S. Pat. Nos. 8,058,486 and 8,084,653, the contents of both of which are incorporated by reference.
One of the major issues discovered in the catalytic conversion of HCO-1230xa to HCFO-1233xf is the degradation and/or deactivation of the catalyst. While not wishing to be bound, it is believed that several factors may contribute to the degradation and/or deactivation of the catalyst, including polymerization within the reactor or coking. Many attempts have been made to address this issue.
U.S. Patent Application Publication No. U.S. 2009/0030244 A1 discloses the use of a stabilizer to prevent polymerization or coking. It has been shown that the use of the stabilizer improves catalyst performance in comparison with catalyzed reactions conducted in the absence of a stabilizer.
U.S. Patent Publication No. US 2011/0155942 A1 discloses the use of a polymerization inhibitor to control polymerization and extend the life of the catalyst. In addition, oxygen is fed to the reactor to extend the catalyst lifetime. For example, in the presence of a polymerization inhibitor, conversion of HCO-1230xa decreased from about 99% to below 50% after running continuously for 100 hours as a result of catalyst deactivation. In the presence of oxygen, the conversion of HCO-1230xa decreased from about 99% to below 50% within 18 hours.
Another process is disclosed in WO 2010/123148 A1, in which catalyst degradation and/or deactivation is avoided by preparing HCFO-1233xf in the absence of a catalyst. This process, however, results in clogging of the reactor due to carbide residue. To remove the carbide residue, oxygen is introduced into the system with the reactants or the reaction is halted to allow oxygen to flow through the system.
Although the use of polymerization inhibitors and supplied oxygen has been suggested for extending catalyst life, these methods are believed to lead to other potential problems. Polymerization inhibitors may have low volatility under the reaction conditions, causing the polymerization inhibitors to plug downstream flow. This build-up of polymerization inhibitors may require more down time of the reactor so that the build-up can be removed. Using supplied oxygen may decrease catalyst activity itself, as well as make purification of the product more difficult. Therefore, the use of polymerization inhibitor(s) or supplied oxygen may pose other issues that affect the efficiency of the reaction.
Thus, there is a need for an economic means of producing hydrofluoroolefins, such as HFO-1234yf, while avoiding the problems discussed hereinabove. The present invention satisfies this need among others.